According to the progress of micro-processing technologies such as a macro-machine/MEMS technology, it is possible to manufacture mechanical vibrators in extremely small sizes. This makes it possible to create the mass of the vibrators themselves very small. Consequently, a vibrator that is so highly sensitive that fluctuation in frequency and impedance characteristics is caused by even a change in mass due to adhesion of very fine substances (e.g., molecules and viruses) on a molecule level is being realized. If the highly sensitive vibrator is used, it is possible to configure a sensor and the like that can detect presence of a very fine substance and an amount of the substance.
A device that detects an amount of a substance according to, for example, a frequency change in a mechanical vibrator is well-known as a QCM (Quarts Crystal Micro balance) sensor. This is a sensor that makes use of a characteristic that, when a substance adheres to a quarts vibrator, a vibrator frequency fluctuates (falls) according to the mass of the adhering substance. The QCM sensor has excellent performance as a mass sensor that measures very small mass. A system for detecting an amount of a substance according to, for example, a frequency change in a mechanical vibrator is also often used as a thickness meter (a vapor deposition monitor).
In such a vibrator, in particular, a micro-mechanical vibrator employing the MEMS technology, since a size thereof has been substantially reduced, a frequency of the vibrator has increased to a GHz level. Moreover, since the micro-mechanical vibrator employing the MEMS technology can be made of an Si material, the technology is being developed into a research aiming at integration with a semiconductor circuit.
As high-frequency filters actively used in personal radio communication devices and the like such as cellular phones, there are a dielectric resonator that mainly realizes a reduction in size and improvement of performance of an electric resonator, a surface wave filter (SAW Filter) that makes use of a characteristic of a sound wave, a quartz crystal filter that uses a mechanical vibration characteristic of a crystal vibrator, and the like. The high-frequency filters are widely used in high-frequency units and the like of cellular phones by making use of characteristics thereof, respectively. However, there is also a strong demand for a reduction in price as well as a further reduction in size and improvement of performance such as an increase in frequency of radio devices. Therefore, in place of these conventional filters, a high-frequency filter of a new system that can be reduced in size and price through integration with a semiconductor integrated circuit (i.e., formation as one-chip) is demanded. Since Si same as a material of a semiconductor is used as a material of a mechanical vibrator manufactured by an MEMS processing technology, the mechanical vibrator is a prospective candidate of the high-frequency filter. Therefore, basic researches for the purpose of an increase in frequency of an MEMS vibrator, an increase in Q value (high quality factor), and the like and applied researches for application to high-frequency filters, transmitters, and the like employing the MEMS vibrator are also actively performed (see, for example, Non-Patent Document 1).
As one type of such vibrators, there is a disc-like vibrator. Basic researches concerning mechanical vibrator of the disc-like vibrator have been performed since long time ago. It may be said that basic researches of vibration modes and the like for defining vibration states of the disc-like vibrator have already been finished.
Non-Patent Document 1: C.T.-C. Nguyen, “Vibrating RF MEMS Technology: Fuel for an Integrated Microchemical Circuit Revolution?.” The 13th International Conference on Solid-State Sensors, Actuators and Microsystems (Transducers '05), Korea, Jun. 5 to 9, 2005